The present invention relates generally to pressure sensing devices, and more specifically, to differential/static pressure sensing devices having high excess pressure protection.
Differential pressure transducers, or transmitters, using a sensor which have only a small displacement over their operating range are well-known in the art as shown in U.S. Pat. No. 3,712,143 issued to Weaver et al. In such a transmitter, the sensor is typically a strain-sensitive semiconductor chip with means for applying pressurized fluids to respective sides of the wafer to produce a physical displacement of the wafer in response to a differential pressure of the fluids. Such differential pressure transmitters include a pair of barrier diaphragms being pressurized with a second input fluid having a second pressure.
A respective interior chamber on the other side of each of the barrier diaphragms contains a respective substantially incompressible fill fluid. These fill fluids communicate through fluid passages with opposite respective sides of the sensor chip, and the chip is displaced or deflected by an amount which is dependent upon the difference between the two pressures of the fill fluids. The sensor chip, in turn, produce an electrical output signal which is dependent upon the displacement and, hence, is representative of a differential pressure of the fill fluids which, in turn, is dependent on the differential pressure of the input fluids.
Such transmitters have usually incorporated pressure overload protection means to prevent the sensor chips from being excessively displaced by overload input differential pressures which could damage the sensor chip. Such overload protection means have been based on providing equal or similar overload protection in both directions of displacement of the sensor wafer.
Another method, as disclosed in U.S. Pat. No. 4,572,000, issued to Kooiman, is the use of sensing diaphragms that are flat when an excessive pressure is applied thereto. Such diaphragms are formed from silicon, quartz, sapphire or other substantially nonhysteretic, brittle, nonmetal.
Yet another method, disclosed in U.S. Pat. No. 4,173,149, issued to Critten et al., uses overload diaphragms that are concavo-convex shaped and are stiff over a given working range, but collapse over a relatively short, high pressure range. Each diaphragm collapses against a housing structure that ultimately bears the excess pressure, thus relieving the pressure sensor. Also included are blocks of material having a low coefficient of thermal expansion relative to the housing material to accommodate increased temperature swings which might distort any pressure readings.